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How do I use extern to share variables between source files?
(19 answers)
Closed 8 years ago.
file1.c => includes file1.h
file1.h => has a struct:
typedef struct {
unsigned char *start;
unsigned int startInt;
}debugPrint;
file1.c => creates a struct object:
debugPrint dp;
file1.c => an int is given into struct:
dp.startInt = 10;
file1.c => has a function:
void function1(debugPrint dp) {
printf("%d", dp.startInt);
}
file2.h => has a function call to file1.c function which is declared before the call:
void function1(void);
function1();
Questions is:
Is it ok that the file2.h calls a function from file1.c
how can i pass the dp.startInt value to file2.h so that the value 10 that was set into dp.startInt in file1.c can be used in the funtion call in file2.h ?
It is needed to be called from file2.h since this file handles dynamic variable exchange between a html page and the file2.h file => data from file2.h function call via file1.c is sent to Html page. But i wont go more into the passing variable to html page since i don't know how it is made. It is a mechanism of openPicus web server example.
But if you know a good solution for this one. i would appreciate it. I'm not so familiar with this kind of code so that is also an issue here :)
But since i think this description is not good enough, here is the files:
file1.c:
#include <stdio.h>
#include <conio.h>
#include <stdlib.h>
#include "test1.h"
// Define printStruct
void printStruct (debugPrint dp) {
printf("%u", dp.startInt);
}
int main ()
{
dp.startInt = 10;
getch();
}
file1.h:
typedef struct {
// For the motorEncoder value
unsigned char filename[20];
char ownMsg[10];
unsigned char *start;
unsigned char *timeInSeconds;
unsigned char *distanceInCm;
unsigned char *numberOfShots;
unsigned char *shutterCompensation;
unsigned char *direction;
unsigned char *cancel;
unsigned char *dutyCycle;
unsigned int cancelInt;
unsigned int startInt;
unsigned int dutyCycleInt;
unsigned int directionInt;
}debugPrint;
// Create struct object called dp
debugPrint dp;
// declare printStruct
void printStruct (debugPrint dp);
file2.h: (this file is totally needed to pass the dynamic values) I didn't put any includes since im not sure how i should include the .h files and from where i should include them.
// Call printStruct function
printStruct(dp);
part of file2.h actual code: (AND YES file2.h A HEADER FILE). FOR ME THIS SEEMS LIKE THE FUNCTIONS ARE FIRST DECLARED AND THEN ONE OF THEM IS USED IN THE H FILE => the HTTPPrint() function from where a function called HTTPPrint_stepCounter(); is called. That function is defined then in file1.c and it just prints some dynamic data to a http page. And as said this is how openPicus has done it and i am just trying to modify it.
void HTTPPrint(DWORD callbackID);
void HTTPPrint_led(WORD);
void HTTPPrint_stepCounter(void);
void HTTPPrint(DWORD callbackID)
{
switch(callbackID)
{
case 0x00000017:
HTTPPrint_led(0);
break;
case 0x00000059:
HTTPPrint_stepCounter();
break;
default:
// Output notification for undefined values
TCPPutROMArray(sktHTTP, (ROM BYTE*)"!DEF", 4);
}
return;
}
void HTTPPrint_(void)
{
TCPPut(sktHTTP, '~');
return;
}
Some tips for someone new to the C language:
There's an important difference between definition and declaration.
Definition is what actually creates the function or variable. Each function must be defined exactly once. Either in a *.c source file, or in a library.
Declaration creates an entry in the symbol table, that says the function or variable exists... somewhere... and here's its data type. Declarations can be duplicated without any effect.
We put function definitions in *.c source files. (And also in libraries, but that's an advanced build topic...)
We put public or extern function declarations in *.h header files.
We put shared extern variable declarations in *.h header files, so that other source units can share the same variable.
We put shared typedef structure declarations in *.h header files, so that other source units can share the same data type.
We do not put variable declarations in *.h header files if they aren't extern, or if they are initialized. The initial value belongs in the *.c file.
Function definitions usually don't belong in a *.h header file, because it's possible in a large project, that the header file could be included (read by the compiler) more than once. That would cause a compiler error, because then there would be more than one definition of that function. Even if it's literally a repeat of the same source code, there can be only one.
The quote about file2.h having a function call to file1.c function is not correct, function1(); could be either a declaration or a function call depending on context:
// declaration of a function named foo
void foo(void);
//
// declaration of a function named bar
// equivalent to declaring void bar(void);
bar();
//
// definition of a function named foo
void foo(void)
{
// call (or invoke) the function named bar
bar();
}
Another small point, about arrays: it's pretty strange to declare an array of one element debugPrint dp[1], since that declaration creates an object that will be referred to as dp[0]. This makes me think you may be trying to avoid the use of pointers... it would be more straightforward to just declare debugPrint dp and then the object is referred to as dp. Arrays make sense if you have more than one related object of the same type, but for just one object, it's a pretty unusual usage.
C is a very flexible programming language that gives free access to lots of low-level tricks. Both a blessing and a curse... For someone just getting started with the language, it's important to read other people's code examples as much as you can, to help learn how things are usually done. There are lots of extremely clever ways to use the language (e.g. Duff's Device) but in most cases, you're better off sticking with the most straightforward and customary way of solving the problem.
See also: What is the difference between a definition and a declaration?
You claim that file2.h contains:
void function(void);
function1();
But these lines refer to two different functions.
This problem is now fixed; both names are function1
If the function1(); appears outside any function, it is a (very sloppy) function declaration, not a function call. If it is inside some function, what is that function definition doing inside the header file. It would need to be an inline function to have much legitimacy.
The problem below is now fixed; the types are consistent.
Additionally, you say: an integer is given into struct: dp[1].startInt = 10;. The compiler complains that you shouldn't assign integers to pointers (since startInt is declared as a pointer, not an int). You need to get your code to compile without such complaints.
... There are two versions of the structure defined, one at the top of the question where startInt is an unsigned int *startInt; and one later on where the declaration is unsigned int startInt. Please make your question self-consistent! ...
This problem has been fixed now; dp is a simple structure.
Also note that you created debugPrint dp[1]; so your initialization is trampling out of bounds; the maximum valid index for the array is 0.
If code in file2.c needs to access the internals of the structure type declared in file1.h, the header file1.h should be included in file2.c. You can declare your dp array in the header too. A header should include other headers only if the functions it defines expose types defined in the other headers. For example, if the structure defined in file1.h included a FILE *db_fp; element, then file1.h should #include <stdio.h> to ensure that the code in file1.h would compile regardless of what else the code using file1.h includes.
Related
I'm writing a library in c and there are some functions I would like to be callable from other c file and some I would like to keep private.
I know that it is possible to hide functions outside of the library file by declaring it static but the same can be achieved by declaring only the implementation on it and leaving it out from the header.
Compare:
a.h
//static int private_routine();
int sample();
a.c
/*static*/ int private_routine(){
}
int sample(){
private_routine();
}
Which practice is best?
To keep functions "private", or better, in scope of a single file, you need to declare them static. Otherwise you can redeclare the same function in other file and still use it.
For example:
#include "a.h" // without declaration of private_routine()
/*extern*/ int private_routine(); // by default every declaration behaves as with extern keyword, which means function is defined elsewhere
int main()
{
private_routine(); // can use here
return 0;
}
EDIT: And as pointed in comments, it's not possible to define function with same name multiple times in one application without declaring it static. If you define a given function name N times, at least N-1 of those must be static.
Can the headers files in C include variables?
I am a beginner in programming; started with C, and I know the importance of precision especially in the first steps of the learning process
Including files is done by the preprocessor before even attempting to compile the code and it simply does text replacement – it puts the contents of the included file in the current unit that is going to be passed to the compiler. The compiler then sees the concatenated output and no #include directives at all.
With that said, technically you can include anything that is valid C code.
The good practice, however, is that only type definitions, #defines, function declarations (not definitions) and data declarations (not definitions) should be in a header. A function declaration is also called a prototype and merely specifies the function signature (its return type, name and parameters). Data declarations look very similar to data definitions, but have an extern storage class specifier and cannot be initialised:
extern int a; // declares "a" but does not define it
extern int a = 0; // defines "a" (initialisation requested), the extern is redundant
int a; // a tentative definition (no initialisation but "a" is zeroed)
Why is defining functions and data in a header file frowned upon? Because at link time, different units that have included the same header files will have the same symbols defined and the linker will see duplicate definitions of some symbols.
Also consider that a header is a kind of a "public" interface for the rest of the project (world?) and not every function that is defined in the source file needs to have a declaration there. It is perfectly fine to have internal types and static functions and data in the source file that never get exposed to the outside world.
Basically in header files, we can declare variables point to be noted only declaration is allowed there, do not define
let me clear.
int a=10; // definition
extern int a; //declaration - it can be used in another file if u include this header file.
you can also define the macro and declare the functions in header file.
Yes, header files may include variable declarations, but you generally don't want to do that because it will introduce maintenance headaches over time, especially as your code gets larger and more complex. Ideally, functions should share information through parameters and return values, not by using such "global" data items.
There are times when you can't avoid it; I haven't done any embedded programming, but my understanding is that using globals is fairly common in that domain due to space and performance constraints.
Ideally, headers should be limited to the following:
Macro definitions
Type definitions
Function declarations
But suppose you do create a header file with a variable declaration, like so:
/**
* foo.h
*/
int foo;
and you have several source files that all include that header1:
/**
* bar.c
*/
#include "foo.h"
void bar( void )
{
printf( "foo = %d\n", foo );
}
/**
* blurga.c
*/
#include "foo.h"
void blurga( void )
{
foo = 10;
}
/**
* main.c
*/
#include "foo.h"
int main( void )
{
foo = 5;
blurga();
bar();
return 0;
}
Each file will contain a declaration for foo at file scope (outside of any function). Now you compile each file separately
gcc -c bar.c
gcc -c blurga.c
gcc -c main.c
giving you three object files - bar.o, blurga.o, and main.o. Each of these object files will have their own unique copy of the foo variable. However, when we build them into a single executable with
gcc -o foo main.o bar.o blurga.o
the linker is smart enough to realize that those separate declarations of foo are meant to refer to the same object (the identifier foo has external linkage across those translation units). So the foo that main initializes to 5 is the same foo that blurga sets to 10, which is the same foo that bar prints out.
However, if you change the declaration of foo to
static int foo;
in foo.h and rebuild your files, then those separate declarations will not refer to the same object; they will remain three separate and distinct objects, such that the foo that main initializes is not the same foo that blurga sets to 10, which is not the same foo that bar prints out (foo has internal linkage within each translation unit).
If you must use a global variable between several translation units, my preferred style is to declare the variable in the header file as extern2
/**
* foo.h
*/
extern int foo;
and then define it in a corresponding .c file
/**
* foo.c
*/
int foo;
so only a single object file creates an instance of foo and it's crystal clear that you intend for other translation units to make use of it. The declaration in the header file isn't necessary for the variable to be shared (the foo identifier has external linkage by simple virtue of being declared in foo.c outside of any function and without the static keyword), but without it nobody else can be sure if you meant for it to be visible or if you just got sloppy.
Edit
Note that headers don't have to be included at the top of a file; you can be perverse and put an #include directive within a function body
void bar( void )
{
#include "foo.h"
// do stuff with foo
}
such that int foo; will be local to the function, although that will likely earn you a beating from your fellow programmers. I got to maintain code where somebody did that, and after 25 years it still gives me nightmares.
1. Please don't write code like this; it's only to illustrate the concept of linkage.
2. The extern keyword tells the compiler that the object the identifier refers to is defined somewhere else.
Here I have two files externdemo1.c and externdemo2.c.In the first file,I have declared and initialized a character array arr at file scope.But I have declared it in the second file externdemo2.c without the extern keyword and made use of it there in the function display(). Here are my confusions arising from it.Please answer these three:
//File No.1--externdemo1.c
#include<stdio.h>
#include "externdemo2.c"
extern int display();
char arr[3]={'3','4','7'};
//extern char arr[3]={'3','4','7'};
//extern int main()
int main()
{
printf("%d",display());
}
//File No.2--externdemo2.c
char arr[3];
int display()
{
return sizeof(arr);
}
1) Why does the program compile fine even though I have declared arr without the extern keyword in externdemo2.c?I had read that the default linkage of functions is external,but I am not sure if that's so even for variables.I only know that global variables have extern storage class.
2) What is the rigorous difference between extern storage class and extern linkage.I badly need a clarification about this.In the first file,where I have defined the array arr,I haven't used the keyword extern, but I know that it has extern storage class by default.But in the second file, isn't there any default extern ,storage class or linkage,about the global variable arr,ie, in externdemo2.c?
3) Check the commented out line in the first file externdemo1.c.Just to test it, I had used the line extern char arr[3]={'3','4','7'};.But it gives the error 'arr' initialized and declared 'extern'.What does this error mean? I have also mentioned a commented line extern int main(),but it works fine without error or warning.So why can we use extern for a function even though a function is extern by default,but not for a variable,like arr here?
Please take some time to bail me out over this.It will clear most of my lingering doubts about the whole extern thing.It will be immense help if you can answer all 3 bits 1),2) and 3). Especially 3) is eating my brains out
Main questions
Basically, because you've included the source of externdemo2.c in the file externdemo1.c.
This is the big question. Because there is no initializer, the line char arr[3]; in externdemo2.c generates a tentative definition of the array arr. When the actual definition with initialization is encountered, the tentative definition is no longer tentative — but neither is it a duplicate definition.
Regarding extern storage class vs extern linkage...Linkage refers to whether a symbol can be seen from outside the source file in which it is defined. A symbol with extern linkage can be accessed by name by other source files in which it is appropriately declared. To the extent it is defined, extern storage class means 'stored outside of the scope of a function', so independent of any function. The variable defined with exern storage class might or might not have extern linkage.
Because it is not defined with the keyword static, the array arr has extern linkage; it is a global variable.
With the commented out line uncommented out, you have two definitions of one array, which is not allowed.
I observe that you must be compiling just externdemo1.c to create a program — the compiler is including the code from externdemo2.c because it is directly included. You can create an object file from externdemo2.c. However, you cannot create a program by linking the object files from both externdemo1.c and externdemo2.c because that would lead to multiple definitions of the function display().
Auxilliary questions
I have placed both files in the [same directory]. If I don't include the second file in the first, then when I compile the first file it gives the error undefined reference to display. Since I have used extern for that function in the first file, isn't the linker supposed to link to it even if I don't include the second file? Or the linker looks for it only in default folders?
There are a couple of confusions here. Let's try dealing with them one at a time.
Linking
The linker (usually launched by the compiler) will link the object files and libraries that are specified on its command line. If you want two object files, call them externdemo1.obj and externdemo2.obj, linked together, you must tell the linker (via the build system in the IDE) that it needs to process both object files — as well as any libraries that it doesn't pick up by default. (The Standard C library, plus the platform-specific extensions, are normally picked up automatically, unless you go out of your way to stop that happening.)
The linker is not obliged to spend any time looking for stray object files that might satisfy references; indeed, it is expected to link only those object files and libraries that it is told to link and not add others at its whim. There are some caveats about libraries (the linker might add some libraries not mentioned on the command line if one of the libraries it is told to link with has references built into it to other libraries), but the linker doesn't add extra object files to the mix.
C++ with template instantiation might be argued to be a bit different, but it is actually following much the same rules.
Source code
You should have a header, externdemo.h, that contains:
#ifndef EXTERNDEMO_H_INCLUDED
#define EXTERNDEMO_H_INCLUDED
extern int display(void);
extern char arr[3]; // Or extern char arr[]; -- but NOT extern char *arr;
#endif /* EXTERNDEMO_H_INCLUDED */
You should then modify the source files to include the header:
//File No.1--externdemo1.c
#include <stdio.h>
#include "externdemo.h"
char arr[3] = { '3', '4', '7' };
int main(void)
{
printf("%d\n", display());
return 0;
}
and:
//File No.2--externdemo2.c
#include "externdemo.h"
int display(void)
{
return sizeof(arr);
}
The only tricky issue here is 'does externdemo2.c really know the size of arr?' The answer is 'Yes' (at least using GCC 4.7.1 on Mac OS X 10.8.3). However, if the extern declaration in the header did not include the size (extern char arr[];), you would get compilation errors such as:
externdemo2.c: In function ‘display’:
externdemo2.c:7:18: error: invalid application of ‘sizeof’ to incomplete type ‘char[]’
externdemo2.c:8:1: warning: control reaches end of non-void function [-Wreturn-type]
Your program looks a bit err. To me the #include "externdemo2.c" line appears invalid.
Following is the correction I have made and it works.
//File No.1--externdemo1.c
#include <stdio.h>
extern char arr[3];
extern int display();
int main()
{
printf("%d", arr[0]);
printf("%d",display());
}
//File No.2--externdemo2.c
char arr[3]={'3','4','7'};
int display()
{
return sizeof(arr);
}
Please follow the below links for better understanding:
Effects of the extern keyword on C functions
How do I use extern to share variables between source files?
Using #include as shown will make both as one file only. You can check the intermediate file with flag -E, as in:
gcc -E externdemo1.c
A little bit of context first. My program features a header, work.h. This header contains a structure, some function definitions, and an extern array of pointers to my base functions.
work.h
typedef struct _foo {
int id;
char something[20];
} foo;
typedef void (*pointer_function)(foo *);
void do_first_to_foo(foo *);
void do_second_to_foo(foo *);
void do_third_to_foo(foo *);
extern pointer_function base_functions[3];
Then a program called work.c with the bodies of the functions, and then the main program main.c. Observe in the header work.h that I have defined prototypes of three functions, and the size of the array is 3, so the pointers on the extern array will point to each one of the 3 functions.
My questions are, how I can associate the pointers of the extern array with the three functions, and second, in which file I need to do it (work.c or main.c).
I understand this association I need to do it in the file work.c, but nothing else.
In work.c:
#include "work.h"
pointer_function base_functions[] = {
do_first_to_foo,
do_second_to_foo,
do_third_to_foo };
Explanation: that array name is a global symbol, and needs to be actually defined in just one compilation unit (.o file produced from .c file). If you do not have it, linker will not find it. If you have multiple copies, linker will complain. You need just one, same as with global exported functions (which are not marked inline).
Edit: Added showing #include "work.h", which is important because it allows compiler to check that extern declaration matches the actual definition. If you leave it out, everything will compile and link without complaints, but you get no indication if there's a mismatch, which could wreak havoc like corrupt other data in memory when variable is used in other compilation units.
I am writing a code in C which has the following basic structure:
Part A: Starting/Init of the main module, calling of various sub-modules and final compliation of the results from the sub-modules.
Part B: Actual execution of the sub-modules.
Now, part A has its own main.c and main.h file
Part B has three modules:
sub1.c/sub1.h
sub2.c/sub2.h
sub3.c/sub3.h
There are a lot of common variables and functions that are used in the sub-modules.
I would like to have a common module which could be #included in all the sub-modules and all the common functions/variables be used. (common.c and common.h)
Now, for the common functions, I can declare them in common.h and then define in common.c and then they could directly be used in all the sub-modules.
But there are a lot of common data variables/members also which i want to 'common' out.
What would be the most efficient way of doing this, so that i could directly use them in all the sub-modules?
In c++, it could just be added to common.h and then could be used with any file that includes common.h but i believe that it is a little different in c.
Could someone please help explain the difference?
thanks
In C or C++:
Should go in a .h:
// declaration, means it's defined somewhere else
// can declare it as many times as you want
extern int yourVariable;
Every object (as in the intermediate file generated during the compilation process for a .c or .cpp file, not an object in OOP) that wants to use a variable needs to know about it (thus have a definition somewhere).
Should go in a .c/.cpp:
int yourVariable = 3; // definition, should only define it once
int yourVariable2; // also a definition
The extern keyword is optional for functions.
int square(int num); // function declaration (.h)
extern int square(int num); // same as above
int square(int num) { return num*num; } // function definition (.c)
In C++:
Should go in a .h:
// this is a declaration
class yourClass
{
int yourVariable;
}
Should go in a .cpp:
int yourClass::yourVariable = 3;
I could be wrong, but I'm not aware of difference between C and C++ in this regard (except that C++ has classes).